Yellow to White Teeth

1- Firstly, open your picture.

2- Now select the Polygonal Lasso Tool with Feather 2px.

3- Zoom in if you need to look closer to avoid mistakes and select the teeth.

4- Go to IMAGE>ADJUSTMENT>HUE/SATURATION or simply click CTRL+U and add these values.

5- Press CTRL+D to deselect the selection and voilá, white teeth.

Here's the difference

Kal El